A network storage apparatus called NAS (network attached storage) is in practical use. NAS has a function of allowing each of plural client devices coupled to a network to access a file stored in the storage apparatus, using a protocol such as NFS (Network File System) or CIFS (Common Internet File System). Some storage apparatuses and file servers using them have a function of limiting utilization of storage resources on a quota basis where the quota is a management unit for a user, group, directory, or the like. PTL 1 discloses a technology used in a network storage system formed of plural network storage apparatuses to limit the utilization of the storage resources.
Storage apparatuses including the NAS vary in the performance level and capacity. Generally, a storage apparatus using a high-speed disk of FC (Fibre Channel), SAS (serial attached SCSI) or the like is high in I/O performance but is costly, which in turn leads to mounting costs for providing large storage capacity. On the other hand, an apparatus using a disk of SATA (serial advanced technology attachment) type, a magnetic tape, or the like is low in I/O performance but is easily provided with large storage capacity at low cost.
A technology called hierarchical storage management (hereinafter called as “HSM”) is known as one of technologies for hierarchical coupling and management of these storage apparatuses. Typically, in the HSM, a high-speed storage apparatus with small storage capacity is arranged at a higher level in the hierarchy, and a low-speed storage apparatus with large storage capacity is arranged at a lower level in the hierarchy. The HSM migrates data from a higher-level storage apparatus to a lower-level storage apparatus by using, as a criterion of judgment for the data migration, the frequency of storage resources utilization by users, or the like, and thus achieves an improvement of the utilization efficiency in the storage system. PTL 2 discloses a technology for data migration from one storage apparatus to another, for use in a hierarchical storage system in which plural storage apparatuses are coupled hierarchically.
Also, a technology called GNS (global name space) is known as one of technologies for unified management of the path names or the like of files stored in a storage system formed of plural storage apparatuses. The GNS enables a user to access a file without knowing which of the storage apparatuses the file is actually stored in. PTL 3 discloses a virtualization technology for a NAS file system including the GNS.